This invention relates to the field of storing and transferring spent nuclear fuel and specifically to a method and system for transferring spent nuclear fuel from a transfer cask to a receiving cask.
In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. Upon removal, this spent nuclear fuel is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. In order to protect the environment from radiation exposure, spent nuclear fuel is first placed in a canister. The loaded canister is then transported and stored in large cylindrical containers called casks. A transfer cask is used to transport spent nuclear fuel from location to location while a storage cask is used to store spent nuclear fuel for a determined period of time.
In a typical nuclear power plant, spent nuclear fuel is loaded into a canister while submerged in a pool of water. The canister is sealed and loaded into a transfer cask while still submerged in the pool. Once loaded with the canister, the transfer cask is used to transport the canister to a receiving cask (i.e., a storage cask or a transport cask). The loaded canister is then transferred from the transfer cask to the receiving cask for either storage or further transport. During transfer from the transfer cask to the receiving cask, it is imperative that the loaded canister is not exposed to the environment.
As a result of this need, the prior method for transferring a loaded canister from a transfer cask to a receiving cask is to raise the transfer cask above the receiving cask and secure the transfer cask atop the receiving cask so that the casks are in a vertically stacked orientation. The transfer cask is adapted so that its bottom can be opened while it remains stacked upon an open receiving cask. Once the bottom of the transfer cask is opened, the loaded canister is lowered from the transfer cask into the receiving cask with a negligible amount of radiation exposure to operations personnel.
Most casks are very large structures and can weigh up to 250,000 lbs. and have a height of 16 ft. or more. As such, stacking a transfer cask atop a receiving cask requires a lot of space a large overhead crane and possibly a restraint system for stabilization. Typically, the transfer of a loaded canister using this stacking method is done inside a 10 C.F.R. 50 structure of a nuclear power plant, which is fully equipped with an overhead crane and radiation containment devices to protect the health and safety of the surrounding communities in the event of a loading mishap. However, numerous nuclear power plants do not possess a 10 C.F.R. 50 qualified staging area that is either large enough to accommodate the stacking of the transfer cask and receiving cask, qualified to support the load of the stacked casks, and/or possesses qualified load handling equipment to make the canister transfer indoors. For such sites, the canister transfer must be completed outdoors using systems and devices that provide the same or greater level of operational safeguards that are available inside a nuclear power plant structure that is fully certifiable for indoor transfer.
It is an object of the present invention to provide a method and system for transferring a loaded canister of spent nuclear fuel from a transfer cask to a receiving cask that requires less vertical space.
Another object of the present invention is to provide a method and system for outdoor transfer of a loaded canister of spent nuclear fuel from a transfer cask to a receiving cask outdoors that provides the same or greater level of operational safeguards that are available inside a fully certified nuclear power plant structure.
Yet another object of the present invention is to provide a method and system for ergonomically completing cask operations necessary for transferring a loaded canister of spent nuclear fuel from a transfer cask to a receiving cask.
Still another object of the present invention is to provide a method and system that greatly enhances the radiation shielding during the transfer of a spent nuclear fuel from a transfer cask to a receiving cask.
Another object of the present invention is to provide a method and system that eliminates personnel lifts and scaffolding that is needed to complete transfer of spent nuclear fuel from a transfer cask stacked atop a receiving cask to the receiving cask.
These objects and others are met by the present invention which in one aspect is a system for transferring spent nuclear fuel to a cask comprising a below grade opening adapted for receiving a cask; a cask support means positioned within the opening, the cask support means capable of vertical movement; and means for vertically moving the cask support means; wherein the cask support means is capable of lowering the cask within the opening.
Preferably, the system further comprises a shell having a cross section, the shell forming walls of the opening wherein the cross section of the shell is slightly larger than the cross section of the cask. The shell and the cask are usually cylindrical.
Also preferably, the means for vertically moving the cask support means is at least two lifting jacks. Moreover, the at least two jacks can be coupled so as to keep the cask support means approximately level during vertical movement. In the preferred embodiment of the system, the number of lifting jacks is three and are located outside the opening and accessible from grade level.
The cask support means has a fully lowered position and a fully raised position. Preferably, when the cask support means is in the fully raised position, the cask support means is below grade. Also preferably, when the cask support means is in the fully lowered position and supporting a cask having a height, at least a major portion of the cask""s height is below grade, with approximately 30 inches of the cask above grade when the cask support means is in the fully lowered position and supporting a cask.
The opening can have a bottom, and the system can further comprise a setdown structure positioned at the bottom of the opening and below the cask support means. In such an embodiment, when the cask support means is in a fully lowered position, the cask support means contacts the setdown structure and the cask support means, and any load being borne by the cask support means is supported by the setdown structure. Preferably, the cask support means is a platform having a center and a top surface wherein the cask support means has a hole near the center and a plurality of cask positioning plates on the top surface.
The system can further include vertical guide rods on which the cask support means can move. Preferably, the vertical guide rods have a top and a bottom, the vertical guide rods being secured at the top so that upon loading the cask support means, the vertical guide rods are in tension.
In another aspect, the invention is a method of transferring a canister of spent nuclear fuel to a cask comprising the steps of: lowering a receiving cask having a height into a below grade opening so that a portion of the receiving cask""s height is below grade level; and transferring the canister to the receiving cask.
The preferred method further comprises placing the receiving cask on a cask support means located within the opening, the cask support means capable of vertical movement; lowering the receiving cask into the opening by lowering the cask support means; aligning the canister above the receiving cask; and lowering the canister into the receiving cask.
Preferably, jacks are used to lower the cask support means wherein the lifting jacks can be coupled so as to keep the cask support means approximately level during vertical movement. The preferred number of lifting jacks is three and the jacks are preferably outside the opening and are accessible from grade level.
Also preferably, a shell having a cross section can be used to form walls of the opening. The shell and the receiving cask can be cylindrical. The cross section of the shell is preferably slightly larger than the cross section of the cask. Also preferably one or more lateral restraints are inserted between the shell and receiving cask.
Preferably, the cask support means has a fully lowered position and a fully raised position, wherein when the cask support means is in the fully raised position the cask support means is below grade level. Also preferably, when the cask support means is in the fully lowered position, at least a major portion of the cask""s height is below grade level. When the cask support means is in the fully lowered position, it is preferable that about 30 inches of the cask be above grade level.
The opening preferably has a bottom with a setdown structure positioned at the bottom of the opening and below the cask support means. It is preferable that when the cask support means is in a fully lowered position, the cask support means contacts the setdown structure, the cask support means and any load being borne by the cask support means being supported by the setdown structure.
Preferably, the cask support means is a platform having a center and a top surface wherein the cask support means has a hole near the center and a plurality of cask positioning plates positioned on the top surface. Also preferably, the cask support means moves along a plurality of vertical guide rods. It is preferred that the vertical guide rods have a top and a bottom, the vertical guide being secured at the top so that any loading from the cask support means results in the vertical guide rods being in tension.